The present invention relates to a method and arrangement in X-ray imaging, in particular three-dimensional imaging, and more especially tomosynthesis.
Tomosynthesis is used to create a three-dimensional picture of a person's body part, e.g. her breast, or an object, using X-rays. Currently, tomosynthesis mammography is available only for research purposes.
Tomosynthesis is essentially a limited form of Computed Tomography or CT. Normally, several projection images, e.g. 5 or 30, are acquired using a modified X-ray system with flat-panel detector and the X-ray source to tube rotated to a unique position for each projection image. Each projection image is essentially a conventional 2-dimensional digital X-ray image of the examined object. The projection images are then combined using special purpose software to produce section or “slice” images representing about a few millimeters thickness of the breast. Viewing the slices in rapid succession provides a volumetric picture of the internal structures of the breast.
Careful optimization of the acquisition process is necessary to obtain a high image quality, a fast image acquisition and keep the radiation doses low (roughly equivalent to a conventional x-ray image). In screening mammography, there are also special requirements related to ergonomics, easy positioning of patients and speed of workflow. For example, open geometry is an advantage, such that the examined object can be reached and seen from several directions. For such reasons, CT cannot substitute tomosynthesis.
EP1428473 discloses a tomosynthesis system for forming a three dimensional image of an object. The system includes an X-ray source adapted to irradiate the object with a beam of X-rays from a plurality of positions in a sector, an X-ray detector positioned relative to the X-ray source to detect X-rays transmitted through the object and a processor, which is adapted to generate a three dimensional image of the object based on X-rays detected by the detector. The detector is adapted to move relative to the object and/or the X-ray source is adapted to irradiate the object with the beam of X-rays such that the beam of X-rays follows in a non arc shaped path and/or a center of the beam of X-rays impinges essentially on the same location on the detector from different X-ray source positions in the sector.
U.S. Pat. No. 6,652,142 discloses how such an tomosynthesis system can be calibrated with respect to geometry, using a set of markers to compute the position of the X-ray source at various positions.
FIGS. 1 and 2 show prior art of a multi-slit X-ray scanner for acquisition of conventional 2-dimensional projection images for digital mammography. The patient is irradiated by a bundle of thin, X-ray beams, each of which is detected by a corresponding line detector. Each beam has a rectangular cross-section, typically 4 cm wide and 50 micrometers across. The narrow beams are created by letting the X-rays pass through a collimator 120, which is a metal plate with several narrow linear apertures, referred to as slits. For each slit, there is one corresponding line detector, which in turn is a silicon array of pixel detectors. The line detectors are arranged to scan virtually the same area of the patient, yielding redundant information and enabling noise reduction. In FIG. 1, the line detectors are mounted in a detector assembly 150. The breast to be irradiated is compressed using a compression plate 140. WO02065209, incorporated herein through reference, discloses a feature for ergonomics during positioning, i.e. the act when inserting the patient's breast. The multi-slit collimator is elevated away towards the X-ray source, hence the operator (nurse) can see and touch from a larger range of directions.
US2005008124 and WO05002443 relate to an apparatus for obtaining tomosynthesis data of an object using multi-slit scanning. Its main advantage relative to prior tomosynthesis systems, is the ability to acquire multiple projection images simultaneously. It comprises a radiation source emitting radiation centered around an axis of symmetry; a radiation detector comprising a stack of line detectors, each being directed towards the divergent radiation source to allow a ray bundle of the radiation that propagates in a respective one of a plurality of different angles to enter the line detector; an exposure area arranged in the radiation path between the divergent radiation source and the radiation detector for housing the object; and a device for moving the radiation source and the radiation detector relative the object essentially linearly in a direction essentially orthogonal to the axis of symmetry, while each of the stack of line detectors is adapted to record a plurality of line images of radiation as transmitted through the object in a respective one of the plurality of different angles.